Device for the transmission by electrical means of oscillations of acoustic frequency



Nov. 4, 1952 2,616,970

H. A. BROOS DEVICE FOR THE TRANSMISSION BY ELE IC MEANS OF OSC ILLATIONSOF ACOUSTIC F UE Filed Dec. 22. 1948 HENRICUS ADRIANUS BROOE BY%/Wy/AGENT INVENTOR.

Patented Nov. 4, 1952 DEVICE FOR THE TRANSMISSION BY ELEC- TRICAL MEANSOF OSCILLATIONS OF ACOUSTIC FREQUENCY Henricus' Adrianus Broos,Eindhoven, Netherlands, assignor to Hartford National Bank and TrustCompany, Hartford, Conn, as trustee Application December 22, 1948,Serial No. 66,814 In the Netherlands February 11, 1948 4 Claims.

This invention relates to devices for the transmission by electricalmeans of oscillations of acoustic frequency, the electrical oscillationsrepresenting the first-mentioned oscillations being set up in afrequency-dependent manner, with the use of filters across at least twochannels, each comprising only one filter, by applying a voltage, ifdesired adjustable, such that for obtaining a spatial impression duringreproduction, if desired via recording of the sound on a support, thestrength of reproduction increases in one channel and decreases in theother channel upon changing from a frequency or group of frequencies toan adjacent frequency or group of frequencies.

The term spatial impression is to be understood to mean the impressionas if the sound originates from one or more sound sources exhibitingspatial extensiveness.

A certain extent of spatial impression is already obtained with a knowndevice in which electrical oscillations of acoustic frequency, comingfrom one channel are divided with the use of some filters which partlyoverlap over an equal number of reproducing channels. The spatialimpression obtained when the loud speakers are arranged in a row withsome spacing and ranged according to increasing or decreasing ranges offrequencies is such that the lowest tones are heard coming from theouter loudspeaker at one side and the highest tones from the outerloudspeaker at the other side, the intermediate loudspeaker reproducingthe intermediate ranges of frequencies. Since the transmissioncharacteristic curves of the filters for two adjacent ranges offrequencies partly overlap, the frequencies of the overlapping regionare reproduced by two loudspeakers, but not in the same strength ratiofor each frequency. The image of the spatial position of such afrequency is thus located between the two loudspeakers and spatiallyvaries in position for each frequency of the overlapping region.

The imperfection of this device resides in the fact that during thereproduction of those frequencies which are not located in theoverlapping portion of the transmission characteristic curve oftwoadjacent filters, but which are passed by only one filter, all thesefrequencies are reproduced at the same spatial area, namely that of theloudspeaker associated with the filter concerned. With chromaticincrease of frequencies a reproduction is thus obtained in which thesound image is at rest at the outer loudspeaker,

then rapidlymoves to a subsequent loudspeaker,

again stops there, during ,a certain time, etc. Consequently, such adevice does not allow of a reproduction in which, according tofrequency, the sound image of a. frequency continuously Varies inposition over that portion of the reproduced range of frequencies whichcontributes to the spatial impression.

In the device according to the invention this disadvantage is obviatedin that the filters exhibit frequency characteristic curves such thatthe whole or substantially the whole ran e of frequencies to bereproduced by at least one of the channels, at least in so far as itcontributes to the spatial impression, is also reproduced by one or moreof the other channels to an extent which contributes to the effect.

The term extent which contributes to the eifect is to be understood tomean that a determined minimum difference in ratio between the soundstrengths of two frequencies or groups of frequencies to be positionedside by side in the sound image is required in each of at least twochannels, viz. a difference in ratio such that the sound images producedof the two frequencies or groups offrequencies occupy preceptibly.different positions in space. As mentioned before, this may be ensuredwith the device according to the invention by causing the strength ofreproduction to increase in at least one of the channels and to decreasein at least one of the other channels upon change in frequency.

As a rule, the aim will be to ensure that the ratio between the sum ofthe strengths of reproduction of a determined frequency in the variouschannels and that of the other frequencies is equal to the ratio betweenthe strengths of the oscillations of the corresponding frequencies upongeneration. Preferably, for example for two channels, use is thus madeof filters such that upon change from one frequency or group offrequencies toan adjacent frequency or group of frequencies, thestrength of reproduction in one channel increases to the same extent asit decreases in the other channel.

Furthermore, if each frequency has. an individual position or the groupsare of equal size, the distance between these frequencies or groups willbe chosen to be equal, so that the filters for each channel are requiredto be such that in the case of two channels and with logarithmicincrease of frequency the voltage transmitted by one filter increases tosuch extent that the sound strength reproduced by the associated channelexhibits a substantially linear increase and, for the other-filter,decreases to such extent that the sound strength reproduced by the otherchannel exhibits a substantially linear decrease.

In a reproducing device comprising three loudspeakers for example theouter loudspeaker, which may be positioned with some spacing, may eachcomprise a filter such that their sound strength increases and decreasesin substantially linear proportion with the logarithm of frequency in arange of, for example, from 500 to 1200 c/sec. The central loudspeakerin this case reproduces the frequencies below 500 c./sec., whichfrequencies, as is well known, do not contribute to the spatialimpression.

The device according to the invention permits inter alia of reproducingwith a spatial impression electrical oscillations which are nottransmitted stereophonically by microphones and which originate, forexample, from a piano and, if desired, have been recorded afterwards inthe form of a sound track. The sound image thereof may be such as toconvey the impression as if it displaces in space, dependently onfrequency, in a similar manner as heard by the player of the instrumenthimself.

It will be evident that the same effect is obtained if an electricmusical instrument is used as a source of oscillations of acousticfrequency. Since stereophonic reproduction of electric musicalinstruments is not possible, a device according to the invention enablesthe sound image of such instruments nevertheless to produce a spatialimpression. The device according to the invention permits of obtaining aspatial impression more particularly of a source of oscillations ofacoustic frequency which exhibits a spatial extensiveness, which issmall with respect to the spatial impression of the sound image thereof.Thus it is possible, for example, that a musical box, all the acousticoscillations of which seem to originate from the same place, provides asound image such that the different tones are heard at different places,hence in such manner that, for example, all tones are heard side byside, in which event the total width of the sound image may be equal tothe distance between the loudspeakers of the reproducing channels.

The invention will now be explained more fully by reference to theaccompanying draw- 'ing showing diagrammatically by way of example, someembodiments thereof.

Fig. 1 shows a device in which each of two channels comprises a filtersuch that a strength of reproduction as shown in Fig. 2 is obtained foreach channel.

Fig. 3 shows a device for three channels. The reproductioncharacteristic curve of this device is shown in Fig. 4. Figs 5 and 6 arevariants of Figs. 3 and 4.

In Fig. 1 reference numeral i designates a channel for the transmissionof electrical oscil lation of acoustic frequency. The oscillations aresupplied to two channels i and 2, each comprising a filter F1, F2,respectively. The filtersare dependent on frequency in such manner thatthe strength of the sound to be reproduced by loudspeakers b isdependent on frequency in a manner as represented in Fig. 2 by the curveA1 for channel 5 and by the curve A: for channel 2.

A determined frequency will thus be reproduced by the loudspeaker ofchannel i and by that of channel 2. The position occupied by thisfrequency in the sound image will thus be located, according to theratio between the strengths of reproduction of the two channels, betweenthe loudspeakers of the channels, the distance between the twoloudspeakers being equal to the total width of the sound image of thewhole of the range of frequencies.

The electrical oscillations of the single channel a may originate eitherfrom a microphone or from a record. As an alternative, they may besupplied by other generating members such, for example, as electricmusical instruments.

Fig. 3 shows the image of a three-channel system for the reproduction ofsound. Here the electrical oscillations of the single channel a aresupplied to the three channels 3, 4, 5, which comsupplied to the threechannels 3, i, 5, which comprise the frequency-dependent filters F3, F4,F5 respectively. As may be seen from Fig. 4, which shows thereproduction curves of the three channels of Fig. 3, the loudspeaker bof channel 3 only reproduces the low frequencies. The higher frequenciesare transmitted with different strengths by the two channels 5 and 5 andthe sound image is thus reproduced dependently on frequency between theloudspeakers of the said channels.

Fig. 5 shows a device which is a variant of Fig. 3. As before, threereproduction channels for obtaining a spatial impression in the reproduction space are shown between the two outer loudspeakers each channelcomprising a filter. The channels 5, l, 6, comprise filters F3, F7, F8respectively such that the sound upon being re-- produced through thesame number of channels via a record produced by members 0 in the formof three tracks, has a strength which is dependent on frequency, asindicated in Fig 6 by A0 for the channel 6 and by A7 and A3 for thechannels 7 and 8. Here the low tones are also divided over two channelsso that each tone occupies its own position. Consequently, upon changefrom a very low frequency to a higher frequency, the strength ofreproduction of the frequencies reproduced by the channels 6 and I willin this device increase in channel I and decrease in channel t.

It is evident that the invention is not limited to the curves of Figs.2, 4 and 6. Other curves may alternatively be used to ensure a certaindesired spatial impression.

Since the electrical oscillations from channel a provided a sound imagehaving a width equal to the distance between the loudspeakers of theouter channels, the device may be used with advantage with instrumentsexhibiting a spatial extensiveness which is small with respect to thespatial impression of the sound image thereof, such, for example, as amusical box.

What I claim is:

1. Electrical apparatus for reproducing, with simulated spacialperspective, audio oscillations, having a given frequency range obtainedfrom a sound source and transmitted through a single channel, saidapparatus comprising a plurality of transmission subchannels, a likeplurality of reproducers disposed in spaced relation, each reproducerbeing coupled to said single channel through a respective subchannel,and a filter network in each subchannel preceding the associatedreproducer, one of said networks having a frequency-responsecharacteristic at which the resultant amplitude of oscillations appliedto the associated reproducer decreases as a function of frequencythroughout a predetermined portion of said frequency range, another ofsaid networks having a frequency-response characteristic at which theresultant amplitude of oscillations applied to the associated reproducerincreases as a function of frequency throughout said predeterminedportion of said frequency range.

2. Electrical apparatus for reproducing, with simulated spacialperspective, audio oscillations having a given frequency range obtainedfrom a sound source and transmitted through a single channel, saidapparatus comprising first and second transmission subchannels, firstand second reproducers disposed in spaced relation and coupled to saidsingle channel through said first and second subchannels respectively,and first and second filter networks inserted in said first and secondsubchannels respectively, said first network having a frequency-responsecharacteristic at which the amplitude of oscillations applied to saidfirst reproducer increases in accordance with frequency through saidgiven range, said second network having a frequencyresponsecharacteristic at which the amplitude of oscillations applied to saidsecond reproducer decreases in accordance with frequency throughout saidgiven range.

3. Electrical apparatus for reproducing, with simulated spacialperspective, audio oscillations having a given frequency range obtainedfrom a sound source and transmitted through a single channel, saidapparatus comprising first, second and third transmission subchannels,first, second and third reproducers disposed in spaced relation andcoupled to said single channel through said first, second and thirdsubchannels respectively, and first, second and third filter networksinserted in said first, second and third subchan nels respectively, saidfirst network having a frequency-response characteristic at which solelythe oscillations within the low-frequency band of said range are appliedwith substantially undisturbed amplitude to said first reproducer, said;

second network having a frequency-response characteristic at which theamplitude of oscillations within the remaining high-frequency band insaid range applied to said second reproducer decreases in accordancewith frequency, said third network having a frequency-responsecharacteristic at which the amplitude of oscillations within saidhigh-frequency band appliedto said third reproducer increases inaccordance with frequency.

4. Electrical apparatus for reproducing, with simulated spacialperspective, audio oscillations having a given frequency range obtainedfrom a sound source and transmitted through a single channel, saidapparatus comprising first, second and third transmission subchannels,first, second and third reproducers disposed in spaced relation andcoupled to said single channel through said first, second and thirdsubchannels respectively, and first, second and third filter networksinserted in said first, second and third subchannels respectively, saidfirst network having a frequency-response characteristic at which theamplitude of oscillations applied to said first reproducer within thelow-frequency band of said given range decreases in accordance withfrequency, said second network having a frequencyresponse characteristicat which the amplitude of oscillations applied to said second reproducerincreases in accordance with frequency within said low-frequency bandand decreases in accordance with frequency within the remaininghigh-frequency band of said given range, said third network having afrequency-response characteristic at which the amplitude of oscillationsapplied to said third reproducer increases in accordance with frequencywithin said high-frequency band.

HENRICUS ADRIANUS BROOS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1, 674, 683 Hahnemann June 26,1928 2, 093, 540 Blumlein Sept. 21, 1937 2, 098, 372 Blumlein Nov. 9,1937 2, 261, 628 Lovell Nov. 4, 1941 2, 352, 696 De Boer et al. June 4,1944

